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| D e c o d e r 1
D e c o d e r 2 2 1 1 P - 1 1 2 P - 1 2 - + + - - û P P P 6 D 1 , n 6 D 2 , n - 1 y 1 y 2 Figure 3.14: Idea of design method No. 1: area between EXIT functions of both decoders should be minimal. Design method No. 2 makes use of the fact that for better SNR values the inverse EXIT function of the second decoder T −1 D 2,n calculated from (λ, ρ) “shifts” to the lower part of the EXIT chart. Hence, if it is feasible for a bad SNR value to shift an EXIT curve to the lower part of EXIT chart, then (λ, ρ) of the resulting EXIT function is expected to improve the performance of the iteration process. Besides, the requirement of an opened tunnel between T D 1,n and T −1 D 2,n will not be affected by this procedure. The input LDPC code parameters for the proposed method No. 2 are (R, d c min , d c max ) where R is the code rate, d c min and d c max are minimum and maximal acceptable check node degree in the Tanner graph of the LDPC code, respectively [7]. Using these parameters and exploiting relations to the code rate, other parameters (d l max , λ 2 min , λ 2 max , d − c , i, λ 2 , λ j , j) can be obtained, from which the last five are related 3.3 Iterative Diversity Reception for Coded OFDM Transmission 57 to the code rate as follows [3]: R = 1 − 1 d − c λ 2 2 + λ i i + λ j j + 1−λ 2 −λ i −λ j d lmax (3.21) Method No. 2 proceeds in an iterative way and utilizes N five-dimensional parameter vectors A = (d c , i, λ 2 , λ j , j ) serving for determination of a specified number of MI values (I e , I a ), and a search is made for the best vector which fulfills a certain condition at each iteration. This condition reads as follows: the sum of distances of MI values corresponding to a vector A for a low SNR value to the predefined straight line should be minimal. The line is placed in the lower part of the EXIT chart and is selected from a group of parallel lines at each iteration according to the procedure presented in the Fig. 3.15. The best vector at each iteration participates in the next iteration and the parameter vector selection takes places using methods of differential evolution [7]. As a result, LDPC code parameters are achieved for which an T −1 D2,n for a certain SNR value is shifted to the preferred area (lower) of the EXIT chart as opposed to initial LDPC code parameters in the first iteration. Figure 3.15: Design method No. 2: if i a i < 10 −9 or there is no improvement in the following iteration, points (I e , I a ) serve as solution. (a) If all points (I e , I a ) lie above the blue solid line, the same line is used in the next iteration (b) If one of the points (I e , I a ) lies below the blue solid line, the blue dashed line will be used in the next iteration. 3.3.4 Performance Evaluation Turbo Diversity vs. MRC Figure 3.16 demonstrates the performance gain of TD compared to MRC for an SNR difference of ∆SNR = 4 dB and ∆SNR = 6 dB. To assess TD in a COFDM framework we have focused on a system design considering the DRM broadcasting standard, mode B, spectrum occupancy 0 [10] and 16-QAM. For LDPC codes at BER = 10 −3 , the performance gain after 25 iterations of a turbo loop and 10 iterations of 58 3 Link-Level Aspects the LDPC decoder compared to MRC with 250 LDPC decoding iterations is in the order of 1.5 dB and 3 dB, for ∆SNR = 4 dB and ∆SNR = 6 dB, respectively. In the case of turbo codes, where only 10 iterations of a turbo loop and 10 iterations of the inner decoder were considered in order to ensure a comparable computational effort, TD outperforms MRC by ca. 1.5 dB. tải về 7.55 Mb. Chia sẻ với bạn bè của bạn:
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